THE EXPANDING ROLE OF VITAMIN D

 

Vitamin D is a prohormone that must be produced in the skin or ingested in a specific quantity to ensure various health outcomes.  Vitamin D has been known to be primarily responsible for calcium and phosphorus homeostasis, but is also is involved in bone mineralization, immune system modulation, suppression of malignant cells, and anabolic activity in the skeleton and renal-cardiovascular system.  Studies have shown vitamin D deficiency to be associated with an increased risk of colorectal and breast cancers and autoimmune diseases. 

Vitamin D has been extensively studied since its dietary insufficiency was identified as the primary cause of rickets in the early 1900’s.  Scientists have discovered that most tissues and cells in the body have vitamin D receptors.  This discovery of more extensive receptors has led to the expansion of the role of vitamin D, and it may play an important role in decreasing the risk of chronic diseases such as multiple sclerosis, colon cancer, inflammatory bowel disease, type 1 diabetes and cardiovascular events.

In recent years, researchers have gained substantial understanding of how vitamin D deficiency can influence pathophysiology and how adequate concentrations possibly can prevent disease.  The discovery that nearly all tissues in the body express the vitamin D receptor (VDR) demonstrates a role for vitamin D in the immune response.   This suggests a potential role for vitamin D in modulating the immune response to various infectious diseases.  

Living at higher latitudes and being prone to vitamin D deficiency increases the risks of developing cancer of the colon, prostate, breast, ovary and several other tissues.  It has been proposed that maintaining adequate concentrations of vitamin D can reduce the risk of these cancers by as much as 30-50%.  Supplementation with vitamin D has been shown to reduce the risk of falling in the elderly.  Inadequate vitamin D, the amount of calcium intake, and living at higher latitudes may be independent contributing factors for the pathogenesis and progression of hypertension and cardiovascular disease.  Active vitamin D is involved in controlling the production of renin, one of the most important hormones for regulating blood pressure.  Vitamin D acts as a negative endocrine regulator for the renin-angiotensin system, which plays an important independent role in hypertension and cardiovascular health.  Additionally, vitamin D can inhibit various aspects of inflammation, which have been established as a key pathogenic mechanism in atherosclerosis.

The best available assessment of vitamin D status is 25(OH)D-3, which is the circulating form of vitamin D.  Vitamin D is stored in adipose tissue.  This measurement does not assess the amount of vitamin D stored in the body, and there is no adequate method of assessing this pool of vitamin D. 

Despite robust skin production, vitamin D toxicity cannot occur from skin production.  Once maximum cutaneous production occurs, additional sun exposure does not result in additional net input to the system.   The same UVB that produces vitamin D in the skin also degrades it, which causes a steady state that limits cutaneous production to a maximum of 20,000 IU/day.  No case reports have ever reported vitamin D toxicity from either sun exposure or from exposure to artificial UVB light. 

Multiple commonly prescribed drugs can inhibit vitamin D absorption, including anti-convulsants, thiazide diuretics, corticosteroids, nicotine, cimetidine, cholesterol-lowering agents (ezetimibe), heparin, warfarin, and certain diet agents (orlistat).  Therefore, vitamin D concentrations should be monitored in individuals taking these medications.

Vitamin D may affect disease processes in the muscle, heart, lungs, skin, bones and gastrointestinal tract. Many vitamin D experts continue to state that the current recommended doses are not high enough to prevent deficiency, which has been reported in 36% of otherwise healthy young adults and up to 57% of general medicine inpatients in the United States.  Recent epidemiologic data documented the high prevalence of vitamin D inadequacy among elderly patients and patients with osteoporosis.   Maintaining adequate serum concentrations of vitamin D likely require higher daily intake and may need to be individualized.  Contact your physician and dietitian for testing and recommendations for supplementation.

Source: Maryann King, MPH, RD, CNSD, LDN